1. Field of Invention
This invention relates to a new and improved method and apparatus for controlling alternating current machines of the synchronous and asynchronous type.
More specifically, the invention relates to a control method and system for reliably controlling the operation of induction motors by closely and effectively controlling the frequency of the rotating electro-magnetic flux produced by the machine while also separately controlling the magnitude.
2. Background Problem
In alternating current machines such as a squirrel cage induction motor, the lack of electrical connection to the rotor prevents direct control of the electrical condition occurring in the rotor. Consequently, the electrical conditions of the rotor can be controlled only indirectly by controlling the nature of the alternating current supplied to the stator of the machine. Known motor control systems for machines of this type generally stabilize operation of the system by using voltage control, and then do a variety of different things with other motor operating characteristics such as the slip frequency. Thus, with known control systems voltage control is employed as the prime stabilizing criterion while frequency normally is considered in a slave or secondary channel of control.
For improved stability in a motor control system, the best way to achieve it is through control of the motor flux. It is known that the electromagnetic flux produced across the stator-rotor gap of alternating current motor is determined primarily by the magnitude and frequency of the alternating current supplied to the stator and the reaction of the rotor. However, what is important is the frequency of the rotating electro-magnetic flux produced across the stator-rotor gap of this machine. This may not always correspond to the frequency of the driving alternating current supplied to the stator of the machine in a system where a current source is employed to supply the motor. If the stator frequency of a current fed induction motor were rapidly changed, the motor will act as a synchronous motor during the resulting transient, i.e. the rotor speed dominates flux and the developed voltage (back emf) may not be synchronized with the current which is controlled by the inverter switching frequency. Therefore a change in the frequency of the supply alternating current (that is the frequency of the current supplied to the stator windings of the motor) may transiently result in generating a rotating electromagnetic flux frequency that is different from the frequency of the supply alternating current, and actually may push power out of the motor even though the motor should not be operating in the generator mode. As a consequence, the alternating current supply cannot necessarily set the frequency of the electromagnetic flux produced in the machine.
The above observed phenomenon does not mean that one cannot build a system that would stabilize the motor, and which uses frequency as a controlling criterion. However, if one is to build a stable control system while using frequency as a controlling criterion, the frequency that should be used is the frequency of the actual electromagnetic flux produced across the stator-rotor gap of the motor because that is the active frequency affecting the operation of the motor.